The recombinant screening program started in 1978 had a very productive second year. We screened approximately 4500 mice for mutation and genetic variation. About 25% of these mice were also checked by mixed lymphocyte reactions as well as skin grafting. During the past year we picked up 16 new recombinants. Four of those were thoroughly analyzed and 2 of them turned out to be intra-l region combinants which has the potential of giving important information on fine structure of the I region. The other recombinants are being established as inbred strains before further analysis will be carried out. Our studies on gene complementations have revealed all the possible allelic combinations which can complement to generate new I region gene products capable of presenting antigen and aiding antigen binding. These results have enabled a further understanding of the mechanism by which I region gene products may be involved in the immune response phenomena. Our collaborative efforts on the functional aspects of 1 region genes has yielded considerable data of significance. In essence, we find that whether an immune response is directed against an antigen on a bacteria, virus, a native protein or a synthetic polypeptide, the regulation of immune response by the MHC gene products might be similar. The final outcome of immune response (resistance to diseases, autoimmune diseases) or immune suppression (susceptibility to diseases, graft acceptance) is the result of a network of interactions of cellular structures coded by the major histocompatibility complex. This basic knowledge gained in the mouse MHC system is already being applied with considerable success in the human and will continue to be a model and homologue for human studies.